In current practice, it is usual to manufacture a welded heat exchanger, in particular where the welding operation consists in brazing, by forming a bundle of tubes and fins between two collector plates, and then joining the various components to each other by the welding operation. To this end, the collector plates are intersected by a succession of said tubes, between which there are disposed, parallel to the tubes, corrugated fins. In this way a stack of tubes and fins is built up.
On the sub assembly formed in this way, a pair of end beams, extending generally parallel to the tubes and fins, are mounted respectively at the beginning and the end of the stack. The various components of this assembly are then wholly or partly covered with welding material, for example brazing metal, which joins all components together and also ensures proper sealing at the junction between the tubes and the collector plates.
Before the welding operation takes place, it has been found that the junction between the tubes and the collector plates tends to give the stack a so-called "barrel-like" shape. This is because, whereas in the region of the collector plates the dimensions of the stack are constrained so as to be substantially the same as the final dimensions of the stack, in the central part of the stack the dimensions are larger, since the various components to be assembled together are covered with welding material and there is nothing to constrain this central part from being able to expand. Thus during the brazing operation, melting of the brazing metal does not allow the stack to assume a conventional shape: on the contrary, it preserves the barrel-like shape of the stack.
It has been proposed that, during the brazing operation, the various components of the stack be clamped together using clamping means having a lower degree of thermal expansion than that of the metal of which the stack is made, in such way as to constrain the cross section of the stack after brazing to a desired value which is substantially the same as that adopted in the region of the collector plate.
However, that proposal has the disadvantage that it cannot conveniently be applied in relation to the shape of the stack. Indeed, it is necessary to provide clamping devices which differ from each other according to their positions on the stack. This can only complicate the assembly of the heat exchanger, and tends to preclude the use of identical clamping means which, because of the barrel-like shape of the stack, will produce either excessive clamping in the middle part of the stack or an insufficient degree of clamping in the region of the collector plates.